1. Field of the Invention
This invention relates to a device which uses one-component magnetic developer called magnetic toner to develop an image bearing surface in an image forming apparatus such as an electrophotographic copying apparatus or a facsimile apparatus.
2. Description of the Prior Art
As an electrophotographic developing method using one-component developer, there is known the powder cloud method using toner particles in the form of spray, the contact developing method in which a uniform toner layer formed on a toner supporting member comprising a web or a sheet is brought into contact with an electrostatic image bearing surface to effect development, the jumping developing method in which a toner layer is not brought into direct contact with an electrostatic image bearing surface but toner is caused to selectively jump to the image bearing surface by the electric field of the electrostatic image, or the magnedry method which uses conductive or magnetic toner to form a magnetic brush which in turn is brought into contact with an electrostatic image bearing surface to effect development.
Of the above-described one-component developing methods, in the powder cloud method, the contact developing method and the magnedry method, toner is brought into contact with the electrostatic image bearing surface irrespective of the image portion (the portion to which toner should originally adhere) and the non-image portion (the background area to which toner should not originally adhere) and therefore, the toner more or less adheres to the non-image portion as well and thus, formation of so-called fog could not be avoided. However, in the jumping developing method (disclosed, for example, in U.S. Pat. No. 4,292,387 which matured from U.S. patent application Ser. No. 58,435), the toner layer is not brought into contact with the electrostatic image bearing surface and development is effected with a gap maintained therebetween and therefore, this method is very effective in preventing the formation of fog. This method, however, utilizes the jumping of the toner by the electric field of the electrostatic image to effect development and the visible image obtained by this method suffers from the following disadvantages.
The principal disadvantage peculiar to the jumping developing method is the problem that the image obtained thereby generally lacks in gradation. In the jumping developing method, the toner jumps only when it overcomes the restraining force of the toner supporting member due to the electric field of the electrostatic image. The force which restrains the toner on the toner supporting member is the resultant force of the Van der Waals force between the toner and the toner supporting member, the adhersing force of the adjacent toner particles, the mirror reflection force between the toner and a developer carrying member (hereinafter also referred to as the sleeve) based on the toner being charged, and the magnetic restraining force by a magnet.
Accordingly, only when the potential of the electrostatic image exceeds a predetermined value (hereinafter referred to as the transfer threshold value of the toner) and the electric field resulting therefrom exceeds said restraining force of the toner, jumping of the toner occurs and adherence of the toner to the electrostatic image bearing surface takes place. Although the force which restrains the toner on the toner supporting member differs in value depending on the individual toners or the particle diameter of the toner even if the toner is one produced and compounded by a predetermined prescription, it seems that such force is distributed narrowly about a substantially constant value, and it appears that correspondingly thereto, the threshold value of the surface potential of the electrostatic image at which said jumping of the toner occurs is also distributed narrowly about a predetermined value. Thus, during the jumping of the toner from the supporting member, the presence of the threshold value causes the toner to adhere to the image portion having a surface potential exceeding this threshold value, while little or no toner adheres to the image portion having a surface potential below the threshold value, with a result that there is only obtained an image poor in gradation in which so-called .gamma.(gamma=the gradient of the characteristic curve of the image density relative to the potential of the electrostatic image) is sharp.
It has been found that if an alternating electric field is applied between the latent image bearing surface and the developer carrying member, said disadvantage is overcome as described, for example, in U.S. Pat. No. 4,292,387, and there can be obtained a fine image of high quality abundant in reproducibility of thin lines and gradation. The effect of the alternating electric field may be explained as follows:
FIGS. 1A and 1B of the accompanying drawings schematically illustrate the developing method described in the aforementioned U.S. Pat. No. 4,292,387, U.S. application Ser. No. 58,435, wherein an alternating electric field is applied between the image bearing member and the developer carrying member. FIG. 1A shows the image portion and FIG. 1B shows the non-image portion. The image bearing member 1 and the developer carrying member 2 are moved in the directions of arrows and pass through developing areas A and B in the meantime. In the developing area A of FIG. 1A, transfer and reverse transfer (i.e. reciprocal movement) of magnetic toner occurs and as the gap between the latent image bearing member 1 and the developer carrying member 2 becomes wider, the electric field therebetween weakens and, in the developing area B, transfer (solid-line arrows) alone occurs and reverse transfer (dotted-line arrows) does not occur. Thus, there is obtained a fine image. On the other hand, in the developing area A' of the non-image portion of FIG. 1B, reciprocal movement of the toner occurs and in the developing area B', transfer of the toner does not occur but reverse transfer alone of the toner occurs and therefore, fog is completely eliminated. Designated by S.sub.1 in FIG. 1 is a developing magnetic pole disposed at the nearest point between the latent image bearing member 1 and the developer carrying member 2.
Now, where the half value width of the magnetic pole S.sub.1 disposed in the developing station is wider than the area in which the toner reciprocates between the latent image surface and the sleeve due to the alternating electric field to develop the latent image (the developing area D=A+B), the magnetic field restraining force strongly acts on the entire developing area and the jumping force of the toner toward the latent image is decreased thereby and thus, the image density is not increased and a good image cannot be obtained.
FIG. 2 of the accompanying drawings shows the relation between the intensity of the magnetic field of the developing magnetic pole S.sub.1 and the developing area D. Curve 3 shows the intensity of the magnetic field of the developing magnetic pole S.sub.1 on the surface of the sleeve. Line 4 indicates the half value width of the peak value C of the developing magnetic pole. In this case, the developing area D is fully contained in the half value width 4. In the jumping developing method shown in the aforementioned U.S. patent, the magnetic restraining force relative to the toner in the area B of the developing area D wherein development is completed greatly affects the image density. Accordingly, where a magnet in which the half value width 4 is thus wider than the developing area D is used as the developing pole, the magnetic restraining force of the toner is strong in the area B wherein development is completed, and this leads to the disadvantage that the image density becomes lower.